1. Field of the Invention
The invention pertains to the field of ratchet tensioners. More particularly, the invention pertains to a ratcheting mechanism for a ratchet tensioner.
2. Description of Related Art
Rack and pawl ratchet chain tensioners are known in the art. Known tensioners include separate rack versions and integral rack versions, where the rack is commonly formed integrally along the side of the piston. This greatly increases the expense of manufacturing the piston.
A rack and washer hydraulic tensioner 10, as disclosed in U.S. Pat. Nos. 5,346,436, 5,577,970, and 6,165,090 and shown in FIG. 1, has a piston 12 mounted in a piston bore 14. The rack 16 engages the piston and is mounted in a separate rack bore 18. The rack 16 is coupled to the piston 12 by a washer or flange 20 attached to the head of the rack 16 and extending over a shoulder on the piston 12. A pawl 22, having teeth and mounted in a pawl bore 24 perpendicular to the rack bore 18, engages the teeth of the rack 16. The rack 16 has significant inertia as a result of its relatively higher mass. Upon initial release when the piston extends to tension the chain and comes to a hard stop, the rack may overshoot, thereby reducing the backlash of the tensioner arm and subsequently inducing higher chain loads than desired.
A hydraulic tensioner 30, as disclosed in U.S. Pat. No. 6,878,082 and shown in FIG. 2, has a rack 32 mounted in a slot 34 of the piston 36. The rack 32 and piston 36 are both mounted in a piston bore 38. Elimination of a rack bore simplifies the manufacture of this tensioner 30. The rack 32 is small and also easily manufactured. The rack 32 is slidable in the rack slot 34, and backlash is provided by a rack slot 34 length that is greater than the length of the rack 32. A toothless pawl 40 mounted in a pawl bore 42 at an angle to the piston bore 38 engages one of the teeth of the rack 32. Although this tensioner has several advantages over the rack and washer tensioner, the closed rack slot 34 requires more care and complexity during assembly of the tensioner, because the rack 32 must be placed into the slot 34 prior to insertion of the piston 36 into the piston bore 38. Also, repeated impacts between the rack 32 and the piston 36 at the end 44 of the rack slot 34 during operation requires the rack 32 and the piston impact area 44 to be highly durable for optimum performance. Additionally, the angle on the pawl bore increases manufacturing costs.
There is a need in the art for a low-cost rack and pawl ratchet tensioner with adjustable backlash.